I get the same error even after going through the basic troubleshooting guide. Tried redownloading it from other servers too. Really hoping this doesn't happen when the DW20 pack hits. D:
EDIT: Just checked for good measure, other modpacks seems to download/install & load up perfectly fine.
More EDIT: The Atlanta server gets through the download up until it tries downloading music files, then it locks up (Doesn't give the same, or any, error).
Yet more EDIT: I've got it working by doing... Nothing. Most of the time it doesn't seem to launch, seems to have done it by sheer luck.